1. Technical Field of the Invention
The present invention relates to a level detector and, more particularly, relates to devices employing level detection of an input signal.
2. Description of the Related Art
Circuits for detecting the level of a signal typically rectify and filter the signal to derive a predetermined measure of the signal amplitude such as, e.g., an average level or a root mean square (RMS) level. Rectifiers such as full wave diode rectifiers are used. After rectification by a full wave rectifier, the output is filtered by a lowpass filter to create a meaningful DC level. Level detectors have been used in applications such as automotive control systems, light and sound intensity detectors, and dynamic range compandors.
A compandor allows the dynamic range of communications signals to be altered for transmission or reception over a communication medium. Such communication medium can be wired or wireless mediums. To decrease an amount of bandwidth or dynamic range required on the communication medium, a communication signal is companded. When companding, the communication signal is compressed before transmission over the communication medium and then expanded (decompressed) after reception over the communication medium. A level detector is used to determine an amount of compression needed on the input signal for transmission over the communication medium. Similarly, a level detector is used for detecting a level of a signal received over the communication system and expanding the signal based upon the detected level.
FIG. 1 illustrates a level detector applicable to a compandor of the prior art. A rectifier 110 rectifies an input signal 105 to produce a full-wave rectified signal 112. Rectified signal 112 is lowpass filtered with a single pole RC time constant determined by a combination of a switched-capacitor 125, switches 115, 120, 130 and 135, and a capacitor 145, producing output signal level 150. Rectifier 110, switched-capacitor 125 and switches 115, 120, 130, 135 are formed in an integrated circuit chip. Capacitor 145 is an off-chip capacitor due to its large capacitance. The large off-chip capacitor 145 must be connected to the integrated circuit chip via a pin connection 140. Placing the large capacitor 145 on the integrated circuit chip would be impractical because its large capacitance would consume a substantial portion of the integrated circuit chip.
The components of the FIG. 1 level detector have been fully integrated to the maximum capacity that was known in the prior art. The switched-capacitor 125 and the switches 115, 120, 130, 135 have been provided to emulate a resistor. An on-chip resistor would consume a greater area of the integrated circuit chip than the switched-capacitor implementation. The switched-capacitor 125 thus avoided placing this resistor off-chip. However, on-chip integration of the large off-chip capacitor 145 has heretofore been impractical.
For most level detection applications, a relatively low 3 dB corner frequency is required for the lowpass filter. For companding applications of voice band signals a corner frequency on the order of 10 Hz is often desired. The 3 dB corner frequency for the prior art shown in FIG. 1 is given by: EQU 3 dB corner frequency=f.sub.s C.sub.125 /2.pi.C.sub.145 (1)
where f.sub.s is a sampling clock frequency controlling the switches of FIG. 1, C.sub.125 is the capacitance value of capacitor 125, and C.sub.145 is the capacitance value of capacitor 145. Sampling of voice signals must be at a sufficiently high rate so as to avoid aliasing, thus a typical sample rate might be 32 kHz. For a desired 3 dB corner frequency of 10 Hz and using a sample frequency of 32 kHz equation (1) produces an extremely large capacitor ratio of approximately 500 to 1. Such a large capacitor ratio becomes impractical or impossible to integrate, and requires a large external capacitor be used. Additionally, with an external capacitor, board leakage currents become an issue and can cause capacitor 145 to become extremely large and range anywhere from 0.01 microfarad to 10 microfarad. Also, extra cost is incurred for both an external capacitor and its needed external connection pin 140.
To reduce the size of electronic devices it is desirable to fully integrate all components such as capacitor 145 on integrated circuit chips. Previous attempts to do so have been unsuccessful due to the aforementioned problems.